In the fields of robots, working machines, automobiles, etc. using electromagnetic motors, the weight reduction of driving systems is demanded. However, because the output densities of the electromagnetic motors depend on their weight, only limited weight reduction is available in actuators comprising the electromagnetic motors. It has thus been desired to develop a small-sized, lightweight actuator capable of providing high output.
Actuators should satisfy such conditions that movable members are displaced to desired positions by a driving force; that the movable members are surely returned to original positions in a nonoperative state; that sufficiently large output is provided to enable the movable members to move even under a large load; etc. Springs are used as pressing members for the movable members to bring them back to the original positions in a nonoperative state. In a case where springs have large resiliency, a large driving force is needed to move the movable member against the spring force. It is thus desired that the springs be deformed by a slight force.
Springs made of superelastic shape memory alloys have recently attracted much attention as resilient members that can be deformed by a slight force, and they have been used for various products such as guide wires of catheters. Thus, attempts have been made to use the shape memory alloy springs for the pressing members of the movable members in the actuators.
However, the conventional actuators comprising shape memory alloy springs, which have a mechanism of deforming the springs by temperature change utilizing the thermoelastic martensitic phase transformation of the shape memory alloys, suffer from slow response because thermal diffusion determines the rate of deformation, though the actuators provide large output and displacement.
It may be contemplated to deform the springs not by temperature change but by a magnetic force. For example, it is known that Ni—Mn—Ga alloy undergo phase transformation in a magnetic field. However it is difficult to form this alloy into springs because of brittleness. JP 11-269611 A proposes, as a magnetic shape memory alloy free from such a difficulty, an iron-based magnetic shape memory alloy such as an iron-palladium alloy containing 27 to 32 atomic % of palladium, and an iron-platinum alloy containing 23 to 30 atomic % of platinum, etc., which are subjected to martensitic phase transformation by external magnetic field energy. Though this shape memory alloy exhibits excellent response because of magnetic control, it disadvantageously needs a larger magnetic field.
Further, JP 10-223430 A proposes, as an actuator utilizing the superelasticity of a shape memory alloy, a magnetic-drive stage comprising a square-shaped, parallel shape memory alloy spring composed of a parallel movable member and a pair of beams perpendicular thereto; a pair of electromagnets disposed on both sides of the parallel spring for driving the parallel spring by a magnetic force; and a permanent magnet for supporting the parallel spring by a magnetic force, each beam comprising hinges with reduced width for easy deformation, and the permanent magnet applying an attractive force to the hinges. Though this magnetic-drive stage can achieve high-accuracy positioning, it provides only small displacement because of using the parallel spring.
As an actuator using the superelasticity of a shape memory alloy, JP 2000-297566 A proposes a driving apparatus comprising a shape memory alloy member that is energized to show superelasticity; a movable member connected to the shape memory alloy member, which is displaced from a stop position to a predetermined active position by energizing and compressing the shape memory alloy member with a spring structure; and a lock mechanism. The movable member of this driving apparatus is maintained at the active position by the lock mechanism. When the driving apparatus is unlocked while the shape memory alloy member is not energized, the movable member is returned to the stop position by a tension coil spring connected to the movable member. This driving apparatus fails to achieve high-accuracy position control, despite positioning by the lock mechanism and the spring.